1. Field of the Invention
This invention relates generally to low-power operating modes of electronic devices, and, more particularly, to a method and apparatus for transitioning between multiple power saving modes of operation.
2. Description of the Related Art
For a variety of reasons, power consumption has become an increasingly significant issue for electronic devices such as computer systems. First, the United States has promulgated a number of regulations regarding power usage or power savings. Further, since many electronic devices, such as laptop computers, draw power from a battery, reducing power consumption can result in significantly longer usage, making these electronic devices more versatile and useful. Additionally, power consumption is related to the amount of heat produced by the electronic device. Reducing power consumption reduces the amount of heat generated, and, thus, reduces the size and complexity of heat sinks, fans, and other structures used to help cool the electronic devices. Accordingly, reducing power consumption can result in reduced costs by eliminating or scaling back the cooling structures. Additionally, reduced heat dissipation generally means that electronic components may be more densely packed, leading to smaller, more compact packages. Moreover, reducing power consumption is environmentally friendly, as it reduces the use of fossil fuels and attendant pollutants.
Accordingly, the electronics industry has been driven to develop a variety of power saving schemes. Computer systems, and other electronic devices frequently have a low-power mode of operation that is implemented using different techniques. These low-power modes primarily take advantage of the fact that most computer systems are not always working at high capacity, but routinely have periods of time where they are essentially idle. Some computer systems reduce power by reducing the power consumed by certain components used in the system during these idle periods of time. For example, many computer systems employ a monitor, which consumes substantial power. Thus, during low-power model the system may turn the monitor off. Likewise, other components, such as a hard disk drive, compact disk (CD) player, random access memory (RAM), and the like may also be turned off. Significant power savings maybe, achieved by removing power from the presently unused peripheral devices. However, when a user attempts to operate a computer system that has its peripheral devices powered-down, a significant delay occurs before the computer system can restore power to the peripherals and begin responding to the requests of the user. Thus, many users bypass or otherwise disable these types of low-power operating modes to avoid these delays.
Other power saving schemes reduce the frequency of a clock signal supplied to components used in the computer system. All other factors being equal, reducing the clock frequency generally proportionally reduces power consumption. Thus, some systems reduce the clock frequency by a significant factor, such as by one-half or more to achieve significant power savings. Normally, however, the various subsystems within a computer system must have a globally synchronized clock signal to insure proper operation. This synchronization is commonly achieved using a phase-locked-loop (PLL) circuit. PLL circuits, however, have a finite frequency range in which they operate with sufficient speed and accuracy to insure that a globally synchronized clock signal may be maintained. Thus, where the clock frequency is changed dramatically, such as by half or more, the PLLs may operate marginally or erratically. Faulty operation of the PLLs may produce unstable operation of the computer system as a whole, causing the system to crash or lock-up. Thus, during transitions from normal to low-power mode, or vice versa, the system may fail, again encouraging the user to disable the power saving feature.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
In one aspect of the present invention, a phase locked loop circuit is provided. The phase locked loop circuit is comprised of a first and second divide-by-N counter, a phase comparator, a voltage controlled oscillator, a clock tree, and a feedback path. The first divide-by-N counter is adapted to receive a first clock signal and provide a second clock signal. The phase comparator has a first and second input terminal and an output terminal. The phase comparator is adapted to compare the phase of signals applied to the first and second input terminals and deliver a signal at the output terminal having a magnitude indicative of a difference in the phases of the signals. The first input terminal is coupled to receive the second clock signal. The voltage controlled oscillator is coupled to receive the phase difference signal and deliver a third clock signal having a frequency responsive thereto. The second divide-by-N counter is coupled to receive the third clock signal and deliver a fourth clock signal. The clock tree is coupled to receive the third clock signal and deliver at least one fourth clock signal. The feedback path is coupled to deliver the fourth clock signal to the second input terminal of the phase comparator.